1. Technical Field
The present invention relates to apparatus and methods used in measuring and monitoring the hematocrit of blood. More specifically the present invention relates to apparatus and methods for measuring the hematocrit of blood by differential geometry and using the hematocrit measuring apparatus to automatically control the parameters of an autotransfusion system.
2. Background Information
Most surgical procedures result in some loss of blood from associated surgical incisions. Injured patients can often experience external and internal bleeding. If blood loss from injury or surgery is substantial, it becomes necessary to replenish lost blood through transfusion.
In many instances, it is possible to collect a patient's blood for use in replacing most or even all of the blood losses. It will be readily appreciated that blood collected from a wound or a surgical site will contain tissue fragments, lysed blood cells, and other unwanted substances. Such blood must be treated for removal of unwanted substances before it is safe for reinfusion into the patient.
The general procedure of collecting a patient's blood, cleansing it, and then returning it to the patient is sometimes referred to as autotransfusion. Where autotransfusion fusion is possible, it is a strongly preferred way of replacing a patient's blood losses. One reason that autotransfusion is so preferred is that it avoids incompatibility problems which sometimes can occur when giving transfusions of blood obtained from someone other than the patient. Use of a patient's own blood to replace blood losses has also become increasingly important in view of issues relating to the safety of replacement blood, such as the prevalence of acquired immune deficiency syndrome (AIDS) or other diseases among blood donors in some locales. Because of these benefits, and others, autotransfusion is often the method of choice for minimizing loss of cellular blood components during diverse procedures ranging from surgery to plasma exchange therapy, and is likely to become increasingly important in the future.
The process of removing blood plasma and other unwanted substances without any cleansing of the blood is commonly referred to as plasmapheresis. Plasmapheresis has long been practiced through use of filters having a pore size large enough to pass plasma and other unwanted substances found in the blood, such as anticoagulant, toxins and components of lysed cells (which, for purposes of brevity and simplicity, shall sometimes hereinafter be referred to collectively as the "waste components" of blood), but small enough to retain intact cells, such as red blood cells, white blood cells and platelets (which shall sometimes hereinafter be referred to collectively as the "cellular components" of blood). Plasmapheresis has also been practiced through use of a centrifuge to separate plasma and other suspended waste components from the denser cellular components, and then removing the plasma and associated waste components.
Simple removal of plasma and associated waste components is not adequate to remove all waste materials associated with blood. It has been found that a more thorough cleansing of blood can occur if the cellular components are washed after the plasma is removed. U.S. Pat. No. 4,631,050 describes a process of autotransfusion utilizing a membrane for filtration to separate waste components from cellular components. That patent describes an initial filtration to remove gross debris, followed by addition of a washing solution to reconstitute the blood, and then subjecting the reconstituted mixture to another filtration step in order to remove remaining waste components. U.S. Pat. No. 4,935,002 describes another autotransfusion apparatus for collecting, processing, and returning blood to a patient during or after surgery. The blood is filtered, washed, and separated from gross particulate refuse.
During the processing of blood for autotransfusion, it is desirable that the hematocrit of the processed blood be maintained in an appropriate range in order to obtain a thorough cleaning while minimizing problems such as clogging the filtration apparatus, damage to the cells, introducing excess solution into the patient, and the other like problems. It is believed that an appropriate range is from about 30% to 55%.
Differential geometry light transmission is a common method for measuring hematocrit. Typically, a number of emitters (especially from LEDs) and detectors are arranged in a predetermined geometric relationship. Light of a known value is emitted and the amount of light received along a given path is measured. These measurements are then applied mathematically to determine the desired parameter.
Unfortunately, while much progress has been achieved in this area, the full nature of light transmission and diffusion through blood under all circumstances has not been completely discovered, and thus is still unclear. Therefore, the mathematical equations used by the devices and methods today are based upon empirical observation by the users of what appears to work well. Consequently, such equations tend to be extremely complicated and require micro-computers to be implemented.
Additionally, after passing through the blood, the light signals received are highly non-linear before conversion, and so require high accuracy Analog/Digital converters, electrical devices used for converting analog signals to discrete digital signals, in order to analyze the wide dynamic range of values encountered by the light detectors.
Further, in the devices used today, it is difficult to precisely control the amount of light actually emitted, especially since the intensity of light output versus drive current degrades as an LED ages.
With regard to the plasma and waste separators in use today, there is no way of continuously measuring the hematocrit of the blood during processing so that the parameters of the processing system can be adjusted to compensate for hematocrit readings outside of a desired range. Therefore, the plasma and waste separators cannot be operated at their optimum levels.